1. Field of the Invention
This invention relates to a local area network (hereinafter called "LAN") line system having a plurality of devices (nodes) spaced away from each other so as to exchange information therebetween, and particularly to a sleep/wake up (hereinafter called a "SLEEP/WAKE-UP") control circuit incorporated into each of nodes which are electrically connected to LAN lines.
2. Description of the Related Art
Each of nodes respectively electrically connected to LAN lines has such a function (hereinafter called a "WAKE-UP function") that each node enters into a mode (hereinafter called a "SLEEP mode") for reducing current to be consumed when each LAN line is not in use and is reset to a communication enable state when it is detected from which node a signal would be output to each of the LAN lines.
Since, however, each node monitors and detects a communication start request to one of nodes even after having been brought to the SLEEP mode, it is necessary to monitor the state of the signal transmitted over each BUS. Accordingly, a power source used for a differential comparator corresponding to a receiving circuit included in each node cannot be turned off. Further, a current of the order of at least several tens of .mu.Amps to several hundreds of .mu.Amps needs to be used up even during the SLEEP mode. Therefore, the more the number of the nodes electrically connected to the BUS lines increases, the more the current consumption of a LAN line system becomes greater, with the result that less power consumption cannot be achieved. When the supply of current to the differential comparator is stopped to realize the less power consumption, a variation in the signal which appears on each of the BUS lines, i.e., the communication start request to one of nodes cannot be monitored and detected.